Low-temperature structural phase transitions of TbB4 and ErB4 studied by high resolution X-ray diffraction and profile analysis

The isostructural rare earth tetraborides TbB₄ and ErB₄ of tetragonal space group P4/mbm undergo structural phase transitions to orthorhombic symmetry around T = 80 K and T = 15 K, respectively. The lattice distortions have been investigated by individual peak profile analysis performed on high-precision X-ray data. The experimental and analytical processing is outlined. The deviations from a tetragonal cell at 4.2 K are 2 × 10⁻² Å for TbB₄ and 3 × 10⁻³ Å for ErB₄. The relative volume change between 300 and 4.2 K is less than 10⁻³ in the TbB₄ lattice and 2.3 × 10⁻³ in ErB₄.The tetragonal to orthorhombic distortions are discussed in the context of the antiferromagnetic phase transitions of TbB₄ at T_N = 43 K and of ErB₄ at T_N = 13 K. The relationship between the structural and magnetic phase transitions differs for the two compounds. In TbB₄, the structural transition, which occurs at a definitely higher temperature than the magnetic ordering, is assumed to be driven by a strong electron-lattice coupling or by an electronic quadrupole-quadrupole interaction. In ErB₄, the structural distortion is attributed to magnetostrictive effects occurring simultaneously with the magnetic ordering process.